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From:   Jann Horn <jannh@google.com>
Date:   Tue, 26 Jun 2018 00:40:46 +0200
Message-ID: <CAG48ez1v3J_VYnrP1RRCf936pvakphGQzvZZWw6tojiHA-BAxw@mail.gmail.com>
Subject: Re: [PATCH] selinux: move user accesses in selinuxfs out of locked regions
To:     Paul Moore <paul@paul-moore.com>
Cc:     Stephen Smalley <sds@tycho.nsa.gov>,
        Eric Paris <eparis@parisplace.org>, selinux@tycho.nsa.gov,
        security@kernel.org, kernel list <linux-kernel@vger.kernel.org>
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On Tue, Jun 26, 2018 at 12:36 AM Paul Moore <paul@paul-moore.com> wrote:
>
> On Mon, Jun 25, 2018 at 12:34 PM Jann Horn <jannh@google.com> wrote:
> > If a user is accessing a file in selinuxfs with a pointer to a userspace
> > buffer that is backed by e.g. a userfaultfd, the userspace access can
> > stall indefinitely, which can block fsi->mutex if it is held.
> >
> > For sel_read_policy(), remove the locking, since this method doesn't seem
> > to access anything that requires locking.
>
> Forgive me, I'm thinking about this quickly so I could be very wrong
> here, but isn't the mutex needed to prevent problems in multi-threaded
> apps hitting the same fd at the same time?

sel_read_policy() operates on a read-only copy of the policy, accessed
via ->private_data, allocated using vmalloc in sel_open_policy() via
security_read_policy(). As far as I can tell, nothing can write to
that read-only copy of the policy. None of the handlers in
sel_policy_ops write - they just mmap as readonly (in which case
you're already reading without locks, by the way) or read.